1. Field of the Invention
The present invention relates to a crystal preparing device which carries out crystal growth of a group III nitride crystal, and a crystal preparing method which carries out crystal growth of a group III nitride crystal.
2. Description of the Related Art
Most of InGaAlN devices (or group III nitride semiconductor devices) which are currently used as ultraviolet, purple, blue, or green light sources are produced by carrying out crystal growth of a group III nitride crystal on a sapphire or silicone carbide (SiC) substrate by using the MOCVD method (organic metal chemical vapor deposition), the MBE method (molecular beam crystal growth method), etc.
When sapphire and silicone carbide are used as the substrate in this case, the substrate and the group III nitride semiconductor greatly differ in the coefficient of thermal expansion and the grating constant, and many crystal defects will be included in the group III nitride semiconductor. Such crystal defects relate directly to the drawbacks which cause deterioration of the device characteristics, for example, short operating life and heavy operation power in a luminescence device.
A sapphire substrate is an insulator, and taking out electrodes from such a substrate as in the conventional luminescence devices is impossible. Thus, it is necessary to take out electrodes from the group III nitride semiconductor. As a result, the area of the semiconductor device becomes large and there is a problem that the semiconductor device must be produced with high cost. Moreover, if the area of the semiconductor device becomes large, another problem of a curvature of the substrate may arise due to the use of the combination of different materials: the substrate and the group III nitride semiconductor.
Moreover, in the case of the group III nitride semiconductor device produced on the sapphire substrate, performing the chip separation by cleavage is difficult and preparing the resonator end surface needed for a laser diode (LD) is difficult.
For this reason, currently, the sapphire substrate is ground or dry etched to a thickness of 100 micrometers or less, and then the chip separation is performed in a manner analogous to that of the cleavage separation so that the resonator end surface is formed. Therefore, it is difficult to perform the formation of the resonator end surface and the chip separation at a single process as in the conventional laser diode (LD), which will increase the manufacturing cost due to complication of the processes.
In order to obviate the problems, there is proposed an improved method in which a group III nitride semiconductor is selectively grew up in a transverse direction on the sapphire substrate, in order to reduce the amount of crystal defects included in the group III nitride semiconductor. Although it is possible to reduce the amount of crystal defects by using the improved method, the above problems related to the dielectric feature of the sapphire substrate and the difficulty of the cleavage separation still remain.
One method to solve the above problems is to use a gallium nitride (GaN) substrate which is of the same material as the crystal which is subjected to crystal growth on the substrate. For this reason, there are proposed various methods of carrying out crystal growth of a bulk GaN through vapor growth, molten metal growth, etc. However, a GaN substrate which has good quality and practical size is not yet realized.
As one method of realizing a GaN substrate, there is proposed a GaN crystal growth method which uses sodium (Na) as flux. See U.S. Pat. No. 5,868,837. In this method, sodium azide (NaN3) and metal Ga are used as raw materials, NaN3 and metal Ga are placed into a reaction container made of stainless steel (the inside dimensions of the container: inside diameter=7.5 mm, length=100 mm) in a nitrogen atmosphere, and the reaction container is held at the temperature of 600-800 degrees C. for 24 to 100 hours, so that a GaN crystal grows.
According to the above-mentioned method, the crystal growth in a range of comparatively low temperatures: 600-800 degrees C. is attained, and the internal pressure of the container is about 100 kg/cm2 which is comparatively low. Thus, the above-mentioned method provides practical crystal growth conditions.
Moreover, there is recently proposed a crystal preparing method in which a mixed molten metal containing alkali metal and group III metal is made to react with a group V raw material including nitrogen, so that a quality group III nitride crystal is prepared. See Japanese Laid-Open Patent Application No. 2001-058900.
Moreover, there is also proposed a crystal preparing method in which a mixed molten metal containing metal Na and metal Ga is put into a crucible, the crucible is placed in a reaction container, and molten metal Na is held outside the crucible and within the reaction container. By raising the vapor pressure of the molten metal Na, crystal growth of a GaN crystal is carried out. See Japanese Laid-Open Patent Application No. 2005-263571.
However, the conventional crystal preparing devices which carry out crystal growth of a GaN crystal have a problem that holding the mixing ratio of metal Na and metal Ga in the mixed molten metal at a controlled ratio during the crystal growth of a GaN crystal is difficult. There may be the case in which the vapor pressure of metal Na is too high so that the vapor is spread from the mixed molten metal to the exterior. Conversely, there may be also the case in which the vapor pressure of metal Na is too low so that the vapor is diffused from the exterior to the mixed molten metal.
Moreover, metal Ga is consumed out of the mixed molten metal with the progress of preparation of crystal growth. As a result, it is difficult to maintain the mixing ratio of metal Na and metal Ga in the mixed molten metal at a controlled ratio stably.